Wavelength conversion in wavelength-division multiplexed (WDM) communication systems would provide significant network performance improvement. Optoelectronic, cross-gain saturation, and cross phase saturation wavelength converters are candidate technologies that have been well characterized, however, they are not “transparent” to either bit-rate or modulation format. Complete transparency is offered only by ultrafast wave mixing techniques-in the present case four-wave mixing (FWM) in semiconductor optical amplifiers (SOAs)

Geraghty, David F.

Koren, Uzi

Lee, Robert B.

Vahala, Kerry J.

English

Caltech Authors - Main

OFC '97 Technical Dig,est Wednesday Afternoon 149 
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WH5 Fig. 3. The maximum number of cascaded switch blocks as a function 
of the switch input power at 10 Gbitk; both with and without regeneration in the 
IWCs. 
increasing the switch size the beneficial effect from the IWCs is still 
significant. The overall maximum number of cascaded switch blocks is 
assessed to eight at a BER floor of lo-'* for an 8 X 8 switch block while 
only two switch blocks in cascade are possible without the regeneration. 
Finally, the corresponding numbers for a 16 X 16 switch block is four 
with regeneration and two withlmt. 
In conclusion, it is demonstrated that the use of IWCs in the 
broadcast and select packet switch block results in an improved cascad- 
ability. Furthermore, it is predicted that successful concatenation of 
eight 8 X 8 and four 16 X 16 switch blocks is possible. 
1. F. Masetti et al., in Proceedings of ECOC'95, 1995, pp. 645-652. 
2. D. Chiaroni et al., in Photonics in Switching, Vol. 12, 1995 OSA 
Technical Digest Series (0 ptical Society of America, Washington, 
D.C., 1995), p. 84. 
G. Soulage et al., in Proceet2ings of ECOC'94, 1994, pp. 451-454. 
M. Schilling et al., in Optiazl Fiber Communication Conference, Vol. 
2, 1996 OSA Technical Digest Series (Optical Society of America, 
Washington, D.C., 1995), 3. 122. 
3. 
4. 
WH6 2:45pm 
~ 
Cascaded wavelength conversions using four-wave 
mixing in semiconductor optical amplifiers 
David F. Geraghty, Robert B. Lee, Kerry J. Vahala, Uzi Koren,* 
Department of Applied Ph.vsics, Mail Stop 128-95, California 
lnstitute of Technology, Pasadena, California 91125; E-mail: 
geraghty@cco. caltech. edu 
Wavelength conversion in wavelength-division multiplexed (WDM) 
communication systems would provide significant network perfor- 
mance improvement.' Optoeletitronic, cross-gain saturation, and cross- 
phase saturation wavelength cc'nverters are candidate technologies that 
have been well characterized: however, they are not "transparent" to 
either bit-rate or modulation format. Complete transparency is offered 
only by ultrafast wave mixing techniques-in the present case four-wave 
mixing (FWM) in semiconductor optical amplifiers (SOAs). TO date, 
Spectrum V a r i a b l e  
~ ~ ~ l ~ ~ e r  At tenuator  
9/1 Tap / 
L/ 
FWM WAVELENGTH CONVERTER 
WH6 Fig. 1. (a) System diagram. (b) Details of the wavelength converter. 
FWM medium is an SOA in Converter 1 and either an SOA or a 4.4-km segment 
of dispersion-shifted fiber in Converter 2. 
demonstrations of noncascaded, SOA FWM wavelength conversion have 
shown negligible degradation to the system performance for bit rates up 
-50 
h 
-50 - I ISecond Stage-Fiber1 
1 5 3 0  1535 1 5 4 0  1545 1550  1555 
Wavelength (nm) 
(b) 
WH6 Fig. 2. (a) BER vs. received power data for the various systems demon- 
strated. (b) Output spectra (0.1-nm resolution bandwidth) of the FWM element 
for the various wavelength converters. 
150 0 Wednesday Afternoon OFC ‘97 Technical Digest 
to 10 G b i t / ~ . ~  However cascading is necessary for implementation of 
converters in scalable WDM networks. Here we present the first demon- 
stration, to our knowledge, of a system utilizing cascaded SOA FWM 
wavelength converters. 
The system, shown in Fig. l(a), uses a bit-error-rate test (BERT) 
system generating a 2.5-Gbit/s PRBS of NRZ ASK data. This signal 
directly drives a distributed feedback laser. The final signal is detected by 
a preamplified receiver, preceded by a variable attenuator and a tap. Our 
wavelength converter, shown in Fig. l(b), combines the input signal with 
a pump laser in a bidirectional coupler (BDC). Mechanical polarization 
controllers (PCs) are used to align pump and signal polarizations. These 
waves are then amplified in a high-power erbium-doped fiber amplifier 
(EDFA), coupled into the FWM medium, and passed through an optical 
bandpass filter (BPF) to suppress the original signal and the pump. 
Several experimental configurations are demonstrated, with corre- 
sponding BER versus received power data and FWM medium output 
spectra shown in Fig. 2. For comparison purposes, the first stage of the 
cascade (Converter 1) is characterized separately. This stage provides 
wavelength conversion of 3 nm utilizing an SOA as the FWM medium. 
The converted signal has an optical SNR of 26 dB into 0.1 nm, as can be 
seen in the upper panel of Fig. 2(b). To enable cascading with Wave- 
length Converter 2, an EDFA-BPF pair is inserted at Converter 1’s output 
to boost the power level of the converted signal. An SOA FWM element 
is used in Converter 2, giving a4-nm shift. This results in a decrease ofthe 
SNR at the output of the second converter to 23.4 dB, as shown in the 
center panel in Fig. 2(b). No additional degradation is observed when 40 
km of fiber (follawed by an EDFA-BPF pair at its output to compensate 
for propagation loss) is inserted between the converters to simulate a 
multitap cascade along a trunk line. 
We repeated the experiment using4.4 km of dispersion-shifted fiber 
as the FWM medium in Wavelength Converter 2. The pump wavelength 
in this case is set for a 14-nm conversion. A SNR of 22.3 dB is obtained, 
shown in Fig. 2(b). Again, no additional degradation is observed when 40 
km of fiber is inserted between the converters. 
*Lucent Technologies, Holmdel, New Jersey 07733 
1. 
2. 
3. 
S.B. Alexander, IEEE J. Lightwave Technol. 11,714-732 (1993). 
S.J.B. Yoo, IEEE J. Lightwave Technol. 14,955-966 (1996). 
R. Ludwig and G. Raybon, Electron. Lett. 4,338-339 (1994). 
WN7 3:OOom 
Comparison of DSF- and SOA-based phase conjugators 
employing noise-suppressing fiber 
H. Geiger, S.Y. Set, R.I. Laming, M.J. Cole, L. Reekie, 
Optoelectronics Research Centre, University of Southampton, 
Southampton SO1 7 IBJ,  U.K.; E-mail: hg@orc.soton.ac.uk 
Dispersion-shifted fiber (DSF) and semiconductor optical amplifiers 
(SOAs) are the most desirable phase conjugators for optical fiber com- 
munication and may be used for dispersion compensation and wave- 
length conversion. Amplified spontaneous emission (ASE) from the 
pump and signal source has been recognized as a limitation to phase 
conjugation, and is usually suppressed by bandpass filtering the pump 
and signal source before combining them.’ More recently, it has been 
suppressed by a bandstop filter at the conjugate wavelength, after com- 
bining pump and signal lighte2 
1554 1538 1538 
digital fast band-pass EDFA 
oscilloscope detector filter 
WH7 Fig. 1. System diagram. 
In this paper, SOA- and DSF-based conjugators are compared using 
identical fiber-grating-based filter networks and 10-Gbit/s nonreturn- 
to-zero ( N E )  intensity-modulated data. We believe we report for the 
first time the use of fiber gratings for efficient ASE filtering and signal 
extraction. We identify ASE from the amplifiers, rather than the conju- 
gation efficiency, as the limitation to phase conjugation in both ap- 
proaches. Although the SOA provides a larger conversion efficiency for 
the same input powers, the optical signal-to-noise ratios (SNRs) are 
similar for both the SOA and DSF conjugators. However, because alarger 
input signal can be employed for the DSF before distortion sets in, the 
best performance is obtained with the DSF. 
A schematic of the system is shown in Fig. 1. The signal light (A, = 
1537 nm) is modulated with a 231 - 1 pseudorandom binary sequence 
via a chirp-free Mach-Zehnder modulator. The signal is combined in a 
3-dB coupler with a pump signal (Ap = 1536 nm). An erbium-doped 
fiber amplifier (EDFA) with output power of -10 dBm increases the 
available power at the phase conjugator. The conjugated signal (A, = 
1535 nm) is extracted from the pump and signal by a circulator and a 
fiber grating centred at A, with a bandwidth of 0.5 nm. The close wave- 
length spacing of the pump and signal is allowed by the sharp cutoff of 
this grating. The circulator output is amplified with an EDFA, and 
out-of-band ASE filtered with a polarization-insensitive bandpass filter. 
The data is received with a 6.4-GHz bandwidth detector and the eye 
diagram examined on a digital oscilloscope. 
Both a GaInAsP SOA (Alcatel A 1901 SOA) and 13 km of DSF are 
compared as phase conjugators. The wavelength of the pump is selected 
to match the zero-dispersion wavelength ofthe DSF (A, - 1535 nm) and 
the pump power chosen below the stimulated Brillouin threshold in the 
DSF.3 Optionally, a second strong grating was used to burn a deep 
spectral hole of 1.2-nm bandwidth at A, to suppress the ASE before the 
conjugator by >40 dB (see spectra in Fig. 1). This grating acts as a 
bandstop filter to suppress the ASE before the DSF or SOA conjugator, a 
similar technique was previously reported for an SOA conjugator.* We 


Cascaded wavelength conversions using four-wave mixing in semiconductor optical amplifiers

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Cascaded wavelength conversions using four-wave mixing in semiconductor optical amplifiers

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